The invention concerns a method for determining the position of a spacecraft based on two vector determinations, wherein sensor data and spin data are determined as output variables for determining the position.
In known stabilization systems, a spacecraft, outfitted with an earth sensor (for direction measurement) and spin wheels that generate a spin target value, can be stabilized around all 3 axes (Whecon principle). The methods employed by such systems make use of the dynamic coupling between rolling and yawing induced by the spin target value in order to stabilize the spacecraft either directly, by correspondingly selected actuator control (jets or wheels), or by using an observer to determine the missing position information (as a rule, the yaw position).
European patent document EP 0 786 132, for example, explains the Whecon principle, on the one hand, and a method for determining the position of a spacecraft as well, in which a decoupling of the individual factors that act upon the spacecraft takes place. Hence, decoupling terms are determined there in order to obtain a decoupling of the roll-yaw coupling terms. This nonetheless represents a relatively high computing expenditure.
U.S. Pat. No. 6,282,467A discloses a method and apparatus for determining the position of a spin-stabilized spacecraft in an inertial reference system. The following steps are carried out: the determination of a spin vector direction in the inertial reference system, based on measured data (in particular the sensor data); the determination of a spin vector in a body-fixed reference system, based on known inertial data; the determination of a direction vector based on single axis sensor data; the determination of a reference direction vector with the aid of an ephemeris model; and the determination of the position of the spacecraft based on the mentioned four vectors and the propagation of the position utilizing sensor data. This method cannot be applied for three axis stabilized spacecraft, and requires in addition a relatively large number of known input variables and sensor data.
It is therefore an object of the present invention to provide a simplified and improved method for determining the position of a spacecraft, in particular a three axis stabilized spacecraft.
This and other objects and advantages are achieved by the method and apparatus for determining the position of a spacecraft according to the invention, based on the determination of direction vectors and spin vectors. Sensor data and spin data are determined as initial variables for determining position, and                a direction vector in a body-fixed coordinate system is measured with the aid of a sensor, as a first vector determination;        a reference direction vector within a reference coordinate system based on the path position of the spacecraft and an orbit model is determined as a second vector;        the overall spin vector of the spacecraft in the body-fixed coordinates system is determined as a third vector; and        the reference overall spin vector of the spacecraft within a reference coordinate system is determined by time propagation of known initial values of the overall spin of the spacecraft or by way of a time tracking of a reference model for the overall spin as a fourth vector.The position of the spacecraft is determined based on the four vectors.        
This method is simpler than the known method according to the state of the art, since utilizing the knowledge of an initial value of the overall spin eliminates the necessity, on the one hand, for an expensive determination of decoupling terms; and, on the other hand, the individual detection steps for determining the position are simplified, as explained below. In particular, basically only one single single-axis vector measurement with the aid of a sensor is necessary, namely for the measurement of the direction vector in a body-fixed coordinate system.
The overall spin can be determined, for example, as a deviation with respect to a reference overall spin. The reference overall spin can be determined, on the one hand, by time propagation of known initial values of the overall spin. For this purpose a measurement or estimate of the overall spin at a specific initial point in time is also required. But the reference overall spin can also be determined by way of a time tracking of a reference model of an overall spin. This corresponds basically to the method of using an observer, which his known in principle from the state of the art. The reference direction vector can be determined basically also (similarly to the reference overall spin) either by time propagation of known initial values of the direction vector or through a time tracking of a reference model of the direction vector.
In one embodiment of the method according to the invention, first of all the rate of rotation or a component of the rate of rotation between a body-fixed coordinate system and the reference coordinate system is determined as an intermediate step of a position determination. In this manner the longitudinal rate component of the speed of rotation or the speed of rotation vector in the direction of the direction vector between the body-fixed coordinate system and the reference coordinate system is first determined based on at least the direction vector, the reference direction vector, and an overall spin that is time-propagated or time-tracked by means of a reference model. The overall spin can moreover be used in a suitable representation, for example, in an orbit coordinate system. With the aid of the longitudinal rate component, the overall spin vector is determined in the body-fixed coordinate system. A transformation matrix that describes the deviations of the actual position of the body-fixed coordinate system of the spacecraft from the target position of the body-fixed coordinate system is determined based on the direction vector, the reference direction vector, the overall spin vector, and the reference overall spin vector; that is, a transformation matrix of the deviations of the actual position of the body-fixed coordinate system of the spacecraft is determined by the reference coordinate system. The reference coordinate system thus represents the target position of the body-fixed coordinate system. The overall spin was utilized again for this method step, so that only the knowledge on the direction vector and on the overall spin is necessary for the overall position determination, which essentially simplifies the method. In addition, in the last method step, the overall spin according to direction and magnitude is not required. The direction of the overall spin alone suffices.
In addition, at least the direction vector and the reference direction vector in particular are used to determine the overall spin vector in the body-fixed coordinate system.
The overall spin is basically determined by whether the spacecraft has spin wheels whose wheel spin constitutes a basic component of the overall spin. The previously described method can be carried out in connection with a spacecraft with spin wheels, even without an explicit measurement of wheel spin. The speed of rotation or components of the speed of rotation can also be determined based on the estimated values or measured values of the wheel spin. Particularly the determination of the longitudinal rate component and the determination of the overall spin vector can occur in addition based on the determined spin vector of spin wheels of the spacecraft.
The method can be carried out in an especially advantageous manner if the deviations of the actual position from the target position are small. This applies in particular when the angle deviations {tilde over (φ)}−BR are smaller than 0.1 wheel in the transformation matrix TBR=1−{tilde over (φ)}−BR are less than 0.1 radius.
An arrangement for determining the position of a spacecraft has the following components:                a device for measuring a direction vector in a body-fixed coordinate system;        a device for determining the path position of the spacecraft;        an arrangement for determining a reference direction vector in a reference coordinate system based on the path position of the spacecraft and an orbit model;        a device for determining an overall spin vector in the body-fixed coordinate system;        an arrangement for determining a reference overall spin vector in the reference coordinate system; and        a device for determining the position of the spacecraft based of the four vectors.        
The aforementioned arrangements can moreover consist of one or more functional components. Such an arrangement serves in particular to implement the method described above, and achieves advantages and possibilities already mentioned.
A further embodiment of the invention has, in addition: a device for determining the longitudinal rate component of the speed of rotation (or, the speed of rotation vector in the direction of the direction vector) between the body-fixed coordinate system and the reference coordinate system
Moreover, the invention may also include an apparatus for measuring or estimating the spin vector of spin wheels of the spacecraft.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.